Glycolysis irreversible reactions

Table 6.5 Irreversible reactions of glycolysis and The enzymes needed in gluconeogenesis to overcome them...

Summary of anaerobic glycolysis. Reactions involving the production or consumption of ATP or NADH are indicated. The irreversible reactions of glycolysis are shown with thick arrows. DHAP = dihydroxyacetone phosphate. 

The most important control step of glycolysis is the irreversible reaction catalyzed by phosphofructokinase (PFK). The enzyme is regulated in several ways ... 

In gluconeogenesis, the following new steps bypass these virtually irreversible reactions of glycolysis ... 

This irreversible reaction is the link between glycolysis and the citric acid cycle. (Figure 17.4) Note that, in the preparation of the glucose derivative pyruvate for the citric acid cycle, an oxidative decarboxylation takes place and high-transfer-potential electrons in the form of NADH are captured. Thus, the pyruvate dehydrogenase reaction has many of the key features of the reactions of the citric acid cycle itself... 

Allosteric interactions. The flow of molecules in most metabolic pathways is determined primarily by the activities of certain enzymes rather than by the amount of substrate available. Enzymes that catalyze essentially irreversible reactions are likely control sites, and the first irreversible reaction in a pathway (the committed step) is nearly always tightly controlled. Enzymes catalyzing committed steps are allosterically regulated, as exemplified by phosphofructokinase in glycolysis and acetyl CoA carboxylase in fatty acid synthesis. Allosteric interactions enable such enzymes to rapidly detect diverse signals and to adjust their activity accordingly. 

In glycolysis each glucose molecule is converted into two pyruvate molecules. In addition, two molecules each of ATP and NADH are produced. Reactions with double arrows are reversible reactions and those with single allows are irreversible reactions that serve as control points in the pathway. 

What do the three irreversible reactions of glycolysis have in common ... 

Gluconeogenesis bypasses the irreversible reactions of glycolysis. Oxaloacetate is an intermediate in this pathway. 

The essentially irreversible reactions that control the rate of glycolysis are catalyzed by... 

Phosphofructokinase. Fructose-6-phosphate is phosphorylated on position 1 by the action of phosphofructokinase (X), which can use ATP, UTP, or ITP. As with other kinases, Mg++ is an essential cofactor. The enzyme contains essential SH groups. This is the second essentially irreversible reaction of glycolysis, since the equilibrium lies far to the side of hexose diphosphate formation. 

Fig. 16.3. Model for glycolysis. Arrows in one direction indicate almost irreversible reactions. Arrows in both directions indicate reactions almost at equilibrium. Broken lines indicate activation (with encircled plus sign) or inhibition (with encircled minus sign) by metabolites. GLU is glucose, HK is hexokinase, F6P is fructose 6 phosphate, PFK is phosphofructose kinase, FDP is fructose 1,6 bi-phosphate, PEP is phosphoenolpyruvate, PK is pyruvate kinase, PYR is pyruvate, and AK is adenylate kinase and LAC is lactate. Prom [7]...

The reverse of the last irreversible reaction in glycolysis is actually two successive enzyme-catalyzed reactions. First pyruvate is converted to oxaloacetate by pyruvate carboxylase, a biotin-dependent enzyme whose mechanism we looked at in Section 24.4. Oxaloacetate is Ihen converted to phosphoenolpyruvate. In this reaction, the 3-oxocarboxyhc acid is decarboxylated (Section 18.17) and the oxygen of the enolate ion attacks the y-phosphorus of GTP (see page 1194). 

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